The present invention generally relates to a powdery developing material and, more particularly, to a developer mix particularly suited for use in electrostatic image development.
The copending U.S. patent application Ser. No. 863,616, filed on Dec. 23, 1977, the invention of which has been assigned to the same assignee of the present invention, discloses a developer mix comprised of a mixture of a magnetic toner with a non-magnetic toner. The present invention pertains to an improvement of such a developer mix as disclosed and claimed in the copending application.
Where the developer mix consisting of the magnetic and non-magnetic toners is used in electrostatic image development to develop an electrostatic latent image, formed on a photoconductive support medium, into a powder or toner image, the non-magnetic and magnetic toners are deposited on the electrostatic latent image respectively by the action of electric charge triboelectrically charged thereto through friction with the magnetic toner and by the action of electric charge reverse in polarity to that of the electrostatic latent image and injected thereinto through an electroconductive dispensing sleeve in a manner similar to that disclosed in the U.S. Pat. No. 3,909,258, patented on Sept. 30, 1975. The subsequent transfer of the powder or toner image so formed on the photoconductive support medium in the manner described above is carried out by the utilization of a corona discharge satisfactorily.
The use of the developer mix of the composition described above has eliminated such problems inherent in the use of the toner-carrier developer mix, one type of two-component developer mix, as resulting from deterioration of the carrier particles which often takes place due to the fact that the carrier particles are not consumed in development of the electrostatic latent image in contrast to the toner particles and, therefore, repeatedly reused. Moreover, the use of the developer mix referred to above is advantageous in that the corona discharge can effectively be used in satisfactorily transferring the powder or toner image from the photoconductive support medium to a sheet of final support material, that is, a copying paper.
In the mixture of the magnetic toner with the non-magnetic toner, it has heretofore been considered appropriate and satisfactory that the magnetic toner particles have an average particle size within the range of 10 to 30 microns. By way of example, the U.S. Pat. No. 4,111,823, patented on Sept. 5, 1978, described that, if the average particle size of the magnetic toner particles is smaller than 10 microns, a satisfactory electrophotographic reproduction of an image can hardly be achieved and, if it is larger than 30 microns, the use of the developer mix will cause an uneven or rough resulting image. The selection of the range of the average particle size of the magnetic toner particles forming a part of the developer mix disclosed in the U.S. Pat. No. 4,111,823 appears to have been made in consideration of the fact that a fine resulting image can be obtained on a copying paper while it has been a practice to use the two-component developer mix having an average particle size within the range of 5 to 30 microns and, in the case of the magnetic toner, the addition of the magnetic toner particles has tended to increase the average particle size.
However, it has been found that the magnetic toner of 10 to 30 microns in average particle size does not necessarily result in satisfactory reproduction of an image of high quality when used in the development process carried under the following particular conditions.
(1) Where an original, the image of which is desired to be reproduced on a copying paper, contains a pale pattern and/or a very fine line, this pattern and/or line of the resultant image reproduced on the copying paper has been found more pale than the original pattern and/or line to such an extent that the reproduced pattern and/or line can hardly be recognizeable.
(2) When the number of revolutions of a magnet unit rotatably housed within the dispensing sleeve in a magnetic brush developing device had been reduced to 1,000 r.p.m. in an attempt to avoid any possible adverse effect which might result from heating evolved by an eddy current, it has been found that electrophotographic reproduction of an area image or a consecutive image results in the reproduced image having a continuous reduction in image contrast gradually decreasing from a front end of the image towards a rear end thereof, thereby lacking a high fidelity reproduction capability.
These problems (1) and (2) described above must be solved by all means to enable the copying machine to produce a satisfactory and acceptable reproduction of images. More specifically, so far as the problem (1) above is involved, in consideration of the fact that the copying machine is frequently used in making reproduction of documents containing letters, emphasis should be placed on improvement of the image concentration even though the reproductivity of a gradation of image may be sacrificed to such an extent that the high fidelity reproductivity of the image gradation will not be lowered, so that the resultant image of high contrast can be obtained in a readily recognizeable form. Therefore, in order to achieve this, the problem (1) above must be solved.
On the other hand, so far as the problem (2) above is involved, to reduce the number of revolution of the magnet unit within the dispensing sleeve is feasible since it can relieve the load imposed on other mechanisms of the copying machine, for example, reduction in motor torque, and since it enables the concurrent use of a drive source not only for driving other movable parts of the copying machine, but also for driving the magnet unit, and in order to assure this, a high fidelity reproduction is necessary. Therefore, the problem (2) above must be solved.